The basic theories of operation and construction of tuning fork gyroscopes are now fairly well known. Typically, tuning fork gyroscopes, as shown in U.S. Pat. No. 5,349,855 entitled COMB DRIVE MICROMECHANICAL TUNING FORK GYROSCOPE, issued to Bernstein et al., include silicon proof masses which are suspended by support flexures above a glass substrate, and comb electrodes which are used for driving an oscillation of the proof masses. Metallic sense electrodes are disposed on the glass substrate below the proof masses for detecting Coriolis acceleration by indicating differential out-of-plane amplitude. Because tuning fork gyroscopes operate in this manner, it is desirable that the amplitude of the oscillation be held at a predetermined constant in order to provide a more accurate output indicating rate.
Prior to the present invention, the amplitude of the oscillating motor of tuning fork gyroscopes has been controlled by a conventional servo loop. In this technique motor position is converted to a proportional voltage by measuring charge variation on a capacitive pick-off which is biased with a DC voltage. The resulting motor position signal is amplified and detected by a full wave rectifier. The rectifier output is then filtered, and the filtered voltage compared against a reference voltage; the difference forming an error voltage. This error voltage is then applied to the motor drive actuator through a loop controller to adjust motor amplitude such that error is driven to zero. However, this particular technique has a potential drawback.
This conventional servo loop may have some instability in the motor position pick-off. The sensitivity of the DC biased pick-off varies slowly over time due to a spurious charge build up on the glass substrate beneath the pick-off electrodes. As this charge accumulates on the glass the sensitivity of the position pick off is modified. In response, the loop controller calls for modified drive force to compensate. The result is a motor amplitude transient wherein amplitude changes over time as substrate charge accumulates. This will result in lower accuracy than is otherwise possible because of the relationship between motor amplitude and Coriolis force in the system.